To achieve a higher recording density in an optical disc recording medium, it is necessary to shorten wavelength of laser used for recording and reproduction while concomitantly increasing a numeric aperture (NA) of an objective lens. However, when tilt occurs between the disc and the laser beam axis as a result of disc rotation or deformation, the focal point of the laser becomes off set from the correct position on the information signal surface of the disc. Primary types of tilting include so-called radial tilt, in which the optical disc recording medium sags of its own weight in a conical manner when the disc is loaded in an optical disc drive apparatus, and so-called tangential tilt, in which the optical disc recording medium tilts in the circumferential direction as a result of the attitude of the optical disc drive apparatus itself or the dimensional precision of loading the optical disc recording medium in the optical disc drive apparatus.
It is necessary to make the recording pits larger to compensate for an offset in the laser beam focal point caused by tilting, and this makes it extremely difficult to increase the recording density. The offset in the laser beam focal point can be reduced if the thickness of the optical disc recording medium substrata is made thinner. However, because disc rigidity drops if the substrata of the optical disc recording medium is made thinner, even greater tilt easily occurs, and not only is the effect of making the substrata thinner impaired, the offset in the laser beam focal point becomes even greater.
Increasing the mechanical strength of an optical disc recording medium by laminating two substrates together is an extremely effective means for preventing tilting of a disc as a result of making the substrates thinner. The recording capacity of a single optical disc recording medium can also be doubled by disposing an information signal surface on two sides of the laminated substrates. For example, when an optical disc recording medium is made by laminating two substrates, it is possible to manufacture a single-sided, single-layer disc OD1 in which a method signal surface is disposed on only one surface of the disc, a single-sided, double-layer disc OD2 in which an information signal surface is disposed in two layers on a single side of the disc, or a double-sided, single-layer disc OD3 in which a single information signal surface is disposed on opposite sides of the disc. An example of a single-sided, single-layer disc OD1 is shown in FIG. 13, a single-sided, double-layer disc OD2 is shown in FIG. 14, and a double-sided, single-layer disc OD3 is shown in FIG. 15. Note that, in each figure, Ls indicates the recording/reproducing laser beam, RS indicates the information recording surface, AS indicates an adhesive layer, and PL indicates a protective layer.
In the case of a single-sided, double-layer disc OD2 as shown in FIG. 14, the laser beam LS2 for recording and reproducing information recording surface RS1 in FIG. 1 must pass through the adhesive layer AS. In addition, a label layer for displaying a title and other disc contents cannot be provided in the case of a double-sided, single-layer disc OD3 shown in FIG. 15.
Methods for manufacturing an optical disc recording medium by laminating a plurality of substrates in this manner can be broadly classified in two based on the coating method of the adhesive for laminating the substrata, i.e., one is a printing method and the other is a spin-coating method. First describing the printing method below with reference to FIG. 11 and FIG. 12, thereafter the spin-coating method is described with reference to FIG. 16, FIG. 17, FIG. 18, and FIG. 19.
An optical disc recording medium manufacturing method using the printing method is simply shown in FIG. 11 and FIG. 12. Using a thermoplastic resin having a high viscosity at room temperature, an adhesive PP is uniformly coated through a screen SP onto the entire surface of a substrate 6 while moving a spatula 60 in a particular direction Ds; and two such substrates are then positioned with the thermoplastic resin PP coated surfaces thereof in mutual opposition, and heated until the thermoplastic resin is becomes soft, and then pressure is applied to press the two substrates together, thereby bonding the two substrates to produce an optical disc recording medium.
However, because the thermoplastic resin is non-transparent, it cannot be used in the manufacture of a single-sided, double-layer disc OD2. In addition, when another substrate is pressed to the thermoplastic resin PF coated onto the entire surface of a substrate, a large number of bubbles necessarily occur between the resin PP and the substrate because of the surface contact therebetween, and the substrates are bonded with these bubbles trapped in the adhesive layer. Because bubbles trapped in this adhesive layer disperse the laser beam and interfere with data recording and reproducing, the printing method cannot be applied to the manufacture of an optical disc recording medium comprising an information recording layer written and read by a laser beam passing through an adhesive layer as in a single-sided, double-layer disc OD2, even if a transparent thermoplastic resin becomes possible in the future.
Moreover, because thermoplasticity is a reversible reaction, exposure to a temperature exceeding the thermoplastic temperature even after substrates are laminated and a finished optical disc recording medium is produced may loosen the adhesive layer and the laminated substrates may warp of their own weight, even resulting in the worse case the top and bottom substrates are displaced or separated. Because the viscosity of the thermoplastic material is high, adhesive that protrudes when the substrates are laminated must be mechanically removed. In addition, the thickness distribution of print layer PP varies between discs in the diametric direction or in the direction Ds of the spatula 60 movement as a result of print coating as shown in FIG. 12. That is, if the distance between substrate 6 and screen SP at diametrically opposite edges of substrate 6 is S1 and S2, the thickness of the adhesive layer on substrate 6 will vary in the diametric direction by .vertline.D1-D2.vertline.. As a result, because the thickness varies in the radial direction at the same circumference, quality problems arise in discs that are rotated for use.
In one printing method a light-permeable light-setting resin is used in place of a thermoplastic resin as the adhesive. In this case, a light-setting resin with a high viscosity similar to the thermoplastic resin is used and printed to the substrates using a method such as shown in FIG. 11 and FIG. 12. The adhesive surface side of the substrate is then exposed to light to slightly set the adhesive, and pressure is applied in a manner pressing the two substrates together, thereby bonding the two substrates and producing an optical disc recording medium. While a light-permeable resin can be used as the adhesive in this method, printing the entire surface again results in bubbles being sealed in the adhesive layer when the substrates are put together. That is, this method cannot be used for the manufacture of a single-sided, double-layer disc OD2. In addition, qualitative problems in the finished optical disc recording medium are involved as a result of variations in adhesive thickness in the radial direction and at the same circumference of the substrate.
A manufacturing method for a single-sided, double-layer disc OD2 using a conventional spin coating method is described briefly below with reference to FIG. 16, FIG. 17, FIG. 18, FIG. 19, and FIG. 20. It should be noted that the optical disc recording medium OD2 is produced by bonding a first substrate 6 and a second substrate 9, which are pre-manufactured using a polycarbonate or other transparent resin by means of an injection molding or other method. A first information recording surface RS1 is disposed on one side of the first substrate 6, and a reflection film is formed on the information recording surface RS1 by such means as sputtering or vacuum vapor deposition. Aluminum is primarily used for this reflection film. A second information recording surface RS2 and reflection film are likewise formed on one side of the second substrate 9. The substrate 6 and substrate 9 thus prepared are then laminated by the below procedure to complete an optical disc recording medium OD2
The work conditions are first set in step #100P. These work conditions primarily include the number of rotations N of the substrate when light-setting resin PP is dropped on the substrate; weight V(g) of the light-setting resin PP dropped on the substrate; weight per second v of the light-setting resin PP dropped on the substrate; rotational speed r1 (rpm) of the substrate when the light-setting resin PP is dropped on the substrate; viscosity .upsilon. (cps) of the light-setting resin PP; rotational speed r2(rpm) of the substrate when the light-setting resin PP is spread; and rotation time t (sec) of the disc when the light-setting resin PP is spread. The intent of these conditions is explained in the following steps. After setting these conditions to specific values, the flow continues to the next step #300.
In the next step #300, as shown in FIG. 16, while the first substrate 6 is spun N times at a low speed rotational speed r1, a light-setting resin PP of Vg used as the adhesive of the laminated substrates is coated in a donut-shaped pattern concentrically to center hole H of the substrate 6 on the side opposite the recording surface RS1 thereof. The task performed in this step is called the "adhesive coating process". The procedure then continues to step #400.
In step #400, as shown in FIG. 17, the substrate 9 is placed on the substrate 6 with its information recording surface RS2 facing the light-setting resin PP. The task performed in this step is called the "substrate matching process". The procedure then continues to step #500P.
In step #500P as shown in FIG. 18, the substrate 6 and substrate 9 are rotated as one body for time t at a high speed rotational speed r2 to spread the light-setting resin PP between the substrate 6 and the substrate 9 by means of a centrifugal force. The procedure then continues to step #700P. This process is called adhesive spreading. It should be noted that any unused light-setting resin PP overflowing from the substrates in this process and in the light-setting resin PP coating process shown in FIG. 16 is recovered, filtered to remove any contaminants, processed to remove any introduced bubbles, and then reused.
In step #700P, as shown in FIG. 19, ultraviolet light is emitted through the second substrate 9 toward the light-setting resin RS1 on the first substrate 6, curing the light-setting resin PP, and fixing the two substrates 6 and 9 integrally laminated. This process is called "bonding". Lamination and production of the optical disc recording medium is thus completed.
It should be noted, however, that even when the work conditions are set in step #100P, the optimum conditions are continuously changing as a result of factors such as changes in ambient temperature and deterioration of the recovered light-setting resin PP. In addition, the amount of light-setting resin PP dropped to the inside circumference part of substrate 6 changes as a result of slight variations in the drop position and volume of the light-setting resin PP, and the temperature of the coating environment. Therefore, even if the light-setting resin PP spreads evenly through the inside circumference part of the disc, the inside circumference edge of the light-setting resin PP layer varies relative to the substrate 6, and good quality cannot be consistently achieved because of varied thickness in the light-setting resin PP layer.
In addition, if the viscosity of the dropped light-setting resin PP is low in the light-setting resin PP dropping step #300, reaction to movement during transportation of the substrate 6 to the next spreading process after resin dropping produces a force that acts on the dropped light-setting resin PP, and causes the light-setting resin PP to spread on the substrate 6 and overflow Because of this spreading of the light-setting resin PP in the transportation direction, variations in the thickness of the light-setting resin PP occur both circumferentially and radially in the transportation direction of the disc, and it becomes difficult to make the light-setting resin PP extend uniformly through the inside circumference part of the disc.
It the light-setting resin PP is thus unevenly distributed and drips from the disc, the substrate 9 cannot be superposed to substrate 6 with linear contact between the substrate 9 and the light-setting resin PP, and the contact area expands. Imposing the substrate 9 on an unevenly distributed light-setting resin PP can easily trap air bubbles between the light-setting resin PP and the substrate 9. Air bubbles remaining in the adhesive layer when the light-setting resin PP is cured cause the laser beam to disperse, and prevent the laser beam from being normally emitted to and reflected from the information recording surface. As a result, air bubbles are fatal flaw in a single-sided, double-layer disc wherein information is recorded and reproduced to the recording surface RS1 of a bottom substrate 6 through an adhesive layer.
In addition, even if bubbles can be removed before curing, that is, when spreading, the light-setting resin PP, variations in the thickness of the light-setting resin PP in the circumferential and transportation directions significantly impair disc recording and read/write precision. This is because the laser is emitted to a data track disposed winding circumferentially to the disc during disc recording and reproducing, and the emission angle of the laser to the track fluctuates irregularly in conjunction with disc rotation.
In addition, when the light-setting resin PP is coated and spread with the spin coating method, the thickness of the light-setting resin PP at the inside circumference of the disc tends to become thinner than at the center of the disc. Therefore, to reduce deviations in the light-setting resin PP thickness, it is necessary to drop the light-setting resin PP as much as possible to the inside circumference side of the substrate 6 so that the light-setting resin PP layer becomes thicker at the inside circumference part of the disc after spreading. However, if the viscosity of the light-setting resin PP is low, the light-setting resin PP will spread on the substrate even if the light-setting resin PP is dropped as much as possible at the inside circumference of the disc, and the light-setting resin PP will protrude from a center hole H of the substrate. Curing the light-setting resin PP in this condition will result in degraded roundness of the center hole H as a result of the light-setting resin PP protruding from the center hole, and will be a factor in disc eccentricity. However, when the thickness of the light-setting resin PP varies in the radial direction of the disc, the light-setting resin PP layer thickness is held uniform in the circumferential direction, and the laser emission angle and aperture on the signal surface are held constant within a single data track revolution unit, and the recording and reading precision can be compensated.
It is therefore necessary to increase the viscosity of the light-setting resin PP in order to prevent bubbles from entering the light-setting resin layer, make the light-setting resin PP reach evenly throughout the inside circumference part of the disc, and reduce variations in middle layer thickness. However, as the viscosity of the light-setting resin PP increases, the time required for filtering and defoaming the recovered light-setting resin PP increases, and the operating efficiency of the apparatus is deteriorated.
When the disc is transported after spreading the light-setting resin, a load is applied to the uncured light-setting resin layer, during transportation, and disc appearance can become worse as a result of bubbles getting into the uncured light-setting resin layer at the inside circumference part of the disc.
There has also not been a simple, effective method for suppressing the offset between the centers of the two laminated substrates to a degree of several ten micrometers with good precision.
Therefore, an object of the present invention is to provide a method and apparatus for laminating production of an optical disc recording medium that is free of the above-described deficiencies of conventional optical disc lamination production methods, free of bubbles in the adhesive layer and resultant obstruction of laser passage, and whereby variations in adhesive layer thickness in the radial and circumferential directions of the optical disc recording medium are suppressed and dimensional precision is assured.
In addition, an object of the present invention is to provide a method and apparatus whereby manufacturing process quality is evaluated based on the precision of the adhesive layer of the manufactured optical disc recording medium, and then the manufacturing conditions are automatically corrected based on the result of the evaluation, and when the manufacturing process quality exceeds the correction capacity, an abnormal state is determined and production is interrupted.